Multilayered co-fired ceramic circuit boards are well known and are fabricated from a stack of layers of ceramic dielectric tape, known as green tape, each layer of which may include a printed metal pattern to form a circuit. Each of the layers has a plurality of small holes, or vias, punched therein which can be filled with a conductive metal so that the various circuit layers may contact each other electrically. The green tape comprises a ceramic and/or glass powder admixed with a suitable organic binder or resin, a solvent, a plasticizer and a surfactant. The manufacturing process for making high density co-fired, ceramic multilayer circuit boards includes stacking a plurality of pre-fabricated green tape layers having via holes punched therein, and circuit patterns printed thereon by applying a conductive ink, i.e., a mixture of a conductive metal powder and a glass powder in a solvent mixture, to fill the vias, and laminating the tape layers together by pressing them in a stack. The stacked layers are then fired at elevated temperatures of over 700.degree. C. This firing burns off the organic material and densifies the glass and/or ceramic used to make the green tape.
Since the fired glass or ceramic circuit boards are fragile, the circuit boards can be attached to one or both sides of a suitable support substrate, or core, to impart additional mechanical strength to the printed circuit board. In the case of a double-sided ceramic-on-metal support substrate, insulated electrical feedthroughs in the metal core can be provided to contact the circuit vias in the multilayer ceramic substrate to further increase the circuit density.
A preferred method of forming such supported multilayer substrates is to form multilayer laminates using green tapes and conductive metal-containing inks in conventional manner, placing the laminates on one or both sides of a suitable prepared support substrate, and firing the composite structure together to the temperature required to remove the organic materials of the green tapes and the conductor inks, to sinter or densify the glass particles of the green tape composition and the metal particles of the conductor inks, and to adhere the resulting multilayer ceramic substrates to the support substrate.
Ordinarily, the removal of organic materials from the green tapes and their subsequent densification results in a large degree of volume shrinkage in the ceramic, up to about 20%, in the x, y and z dimensions. However, since the support substrate in the above described supported printed circuit boards does not undergo any densification shrinkage, the large shrinkage of the green tapes, especially in the x and y dimensions, will lead to substantial problems such as non-adherence to the support substrate and serious misalignment between the vias in the multilayer ceramic layers and the electrical feedthroughs on the support substrate they are supposed to contact. Thus some method of restraining the shrinkage of the green tape layer, at least in the x and y lateral dimensions, must be provided for.
One method for restraining the lateral shrinkage of the green tape laminate fired on a support substrate is to provide a bonding layer between the laminate and the support substrate that is capable of adhering the laminate to the support substrate before the onset of densification shrinkage in the laminate. A glass bonding layer has been proposed to accomplish this, for a specific set of metal core and ceramic composition, see U.S. Pat. No. 5,277,724 to Prabhu. The glass bonding layer must be adherent to the support substrate and to the ceramic or glass of the laminate. Thus when the green tape laminate placed on the bonding glass layer is fired, the glass bonding layer suppresses shrinkage in the x and y lateral directions, with the result that almost all of the shrinkage in the green tape layers occurs in the thickness, or z, dimension. Thus vias and contacts in the multilayer ceramic stack and in the support substrate remain in alignment with each other during firing. Cherukuri et al, U.S. Pat. No. 5,256,469, disclose Pb-Zn-Ba borosilicate glasses as bonding glasses suitable for a variety of support substrates and magnesium-containing green tape ceramics of the MgO-B.sub.2 O.sub.3 -SiO.sub.2 type. These ceramics have a thermal coefficient of expansion for the fired ceramic of 90-130.times.10.sup.-7 /.degree. C.
When a green tape stack is to be laminated to both sides of a ceramic or metal support substrate, to further increase the density of devices on the circuit board, the problem of shrinkage becomes critical. The multilayer circuits on both sides of the support substrate, which are connected through via holes in and contact pads on the support substrate, must remain in registration with these via holes and contact pads. Since the ceramic or metal support or core does not shrink perceptibly during firing, the amount of shrinkage of the glass/ceramic green tapes that can be tolerated in the x and y directions can only be about 1% or less in order to maintain registration between the various layers and the vias in the support substrate.
Thus it is desired to provide a bonding layer to bond a forsterite-cordierite-type glass/ceramic to a ceramic or metal support substrate, such as a nickel plated Cu/Mo/Cu (13/74/13) substrate or a Kovar substrate, that will prevent x-y shrinkage greater than 1% between the support substrate and the glass/ceramic during sintering of the green tape at temperatures up to about 950.degree. C.